The preferred embodiment of the present invention is particularly directed to a system for moving an ink jet printer back and forth along a guide bar for scanning a print medium on which an image is printed. In general terms, the underlying invention is applicable to a system for moving reciprocatingly a mass along a predetermined surface. It will be understood in the following description that although reference is made to the ink jet printer environment, the invention is not limited to this application.
A carriage motion controller is generally a four state device. One state is movement left at a constant velocity. A second state is left reversal in which the velocity of the carriage is reversed. A third state is movement right at a constant velocity. The fourth state is right reversal in which the velocity of the carriage is again reversed. Conventional control schemes are variously described in "Closed Loop Operation of Step Motors", Warner Electric's Guide to Selecting and Controlling Step Motors, 1979, pages V-1 to V-21; Regnier et al., "Starting Brushless DC Motors Utilizing Velocity Sensors", Incremental Motion Control Systems and Devices, Fourteenth Annual Symposium of the Incremental Motion Control Systems Society, June 1985, pp. 99-107; and Petrick, "Microprocessor Stepper Motor Drive", U.S. Pat. No. 4,648,026 issued Mar. 3, 1987.
The constant velocity states are maintained by controlling a drive motor by a velocity servo. During reversals, the drive motor is driven by a constant current source. With a DC drive motor, constant current results in constant torque, which, ideally, produces constant deceleration and acceleration. As also used herein, deceleration is considered negative acceleration, with acceleration being a general term encompassing both positive and negative values.
This scheme does not compensate for the effects of friction forces on the printer carriage. Friction assists deceleration but hinders acceleration. Thus, the theoretical expectation of accomplishing deceleration and acceleration during velocity reversal over equal travel distances of the carriage is not satisfied. In one application, for instance, it has been found that the carriage decelerates to zero velocity over a distance of about 0.75 inches. The carriage accelerates to the design speed of 33.33 inches per second over a distance of about 1.25 inches. This requires that the printer be wide enough to accommodate the longer distance at each end of the printer carriage scan in order to reach design speed before beginning printing.
It is therefore desirable to have a more narrow printer in order to reduce overall size as well as cost. More specifically, it is desirable to accelerate the carriage at an actual rate that is the same as the actual rate of deceleration.